abominableman
- Mar 19, 2010
- 3
- Joined
- Mar 19, 2010
- Messages
- 3
Hi everyone,
Check out the pdf attachment. Looks like a perfect, simple solution, but I do not know how to calculate the values for the resistors.
I'm also building an array of LEDs - 8 rows of 6 - powered by a 24V DC power supply.
I need 20mA through each leg
LED Forward voltage = 3.2V
Can anyone figure out the resistor values for me and for huntxtrm?
Thanks!!!
Dan
I found the diagram at the on Semiconductor website (can't post links yet) Here's what they say:
When the circuit operates properly and all the LEDs are
running, the three sense resistors have about 1.25 V across
them, which turns the transistor switches ‘on’. This connects
all three sense resistors back to the Vadj pin allowing the
proper current to go through each leg. If one string opens up,
the sense resistor for that leg won’t have any voltage across
it, turning ‘off’ the transistor and disconnecting its sense
resistor from the Vadj pin. Therefore, the other two LED
strings are unaffected by the fault. This same scheme can be
expanded to accommodate as many LED strings as needed.
Check out the pdf attachment. Looks like a perfect, simple solution, but I do not know how to calculate the values for the resistors.
I'm also building an array of LEDs - 8 rows of 6 - powered by a 24V DC power supply.
I need 20mA through each leg
LED Forward voltage = 3.2V
Can anyone figure out the resistor values for me and for huntxtrm?
Thanks!!!
Dan
I found the diagram at the on Semiconductor website (can't post links yet) Here's what they say:
When the circuit operates properly and all the LEDs are
running, the three sense resistors have about 1.25 V across
them, which turns the transistor switches ‘on’. This connects
all three sense resistors back to the Vadj pin allowing the
proper current to go through each leg. If one string opens up,
the sense resistor for that leg won’t have any voltage across
it, turning ‘off’ the transistor and disconnecting its sense
resistor from the Vadj pin. Therefore, the other two LED
strings are unaffected by the fault. This same scheme can be
expanded to accommodate as many LED strings as needed.